The invention relates to a method for the nondestructive testing of pipes of ferromagnetic steel by means of stray flux.
The known stray flux test is used in pipes of ferromagnetic steel for detecting longitudinally aligned or transversally aligned near-surface discontinuities, like, e.g., cracks, that could otherwise not be detected at all or only with high inaccuracy when using other test methods in a cost-intensive and time-consuming manner.
This method detects e.g. cracks which extend from the surface of the pipe at least by about 0.3 mm (Nondestructive Evaluation, A Tool in Design, Manufacturing, and Service, CRC Press 1997).
Known prior art measuring methods for detection of near-surface flaws on the inner or outer sides of pipes involve the application of constant field magnetization.
In contrast to alternating field magnetization which is used, for example, in bar material and which permits only detection of external flaws of pipes, the constant field magnetization allows also detection of flaws on the inner surface of pipes.
The stray flux testing with constant field magnetization utilizes the effect that the induction flux density increases in the area of a flaw, whereby the magnetic field lines are disturbed by external or internal flaws in their otherwise straight expansion, thereby forming a so-called stray flux. This stray flux, which exits from the pipe surface, is used for the detection of flaws.
The magnetic stray flux is measured, e.g., with induction coils, Hall probes, or GMR sensors, which are arranged in a test head.
A magnetic field is applied at a right angle when testing for longitudinal flaws, and along the pipe axis when testing the pipe for longitudinal transverse flaws. The entire pipe surface is then inspected for flaws during the course of a continuous and quality-assuring pipe production.
In order to measure the entire surface during testing of the pipe for longitudinal flaws, it is necessary to move the pipe and test head relative to each other in a helical manner. This may be realized, e.g., by transporting and simultaneously rotating the pipe in a longitudinal direction under the fixedly arranged test head, or with a test head that orbits in a circular manner about the pipe while the pipe moves only in the longitudinal pipe direction, or by only rotating the pipe about a test head which is moveable in longitudinal pipe direction, or by moving the test head in a helical manner about the pipe while the pipe is at a standstill.
The test for transversal flaws typically involves a probe ring which is fixedly positioned about the pipe, with the pipe moving under the probe ring in longitudinal direction. When testing for a combination of longitudinal and transversal flaws, the movement patterns that are possible for testing for longitudinal flaws may also ensue.
The processed signals can then be used for sorting and marking the pipes and the test results can be recorded.
The test device is aligned by using a groove as test flaw which has been introduced onto a reference workpiece exactly in perpendicular relation to the testing or magnetic field direction. As soon as the test flaw is no longer perpendicular in relation to the magnetic field, the signal amplitude decreases progressively as the angle increases.
Although the known stray flux test is able to reliably detect possible discontinuities on the pipe surface, an association of the flaw signals to the outer and inner surfaces of the pipe, i.e. a separation of flaws when the flaws is oriented at an angle with respect to the magnetic field direction, is not possible with this method with sufficient certainty.
A separation of flaws according to outer or inner surface flaws on the pipe is desirable for many reasons even when flaws extend at an angle to the magnetic field. Flaws on the outer or inner surface of the pipe may have different origins, caused, for example, by the preceding production steps (defective internal tool or rollers) or by flaws in the source material.
Furthermore, an early fault localization and fault recognition with respective corrective measures may assist in the prevention of high failure and reworking rates. Depending on the pipe diameter, it may no longer be possible to rework flaws on the inner surface of the pipe, so that these pipes have to be sorted out as rejects at any rate.
According to experiments performed in-house, even a frequency analysis of the measured signals is not sufficient for an accurate association of flaws, since the measured frequencies are located close to one another and a type of “background noise” is additionally superimposed. This effectively coherent background signal may have various causes, e.g., wall thickness variations caused by rolling.
For this reason, according to WO 02/095383 A2, an attempt is made to minimize this background signal by forming a local difference between the measured signals obtained from at least two single probes located in the same plane.
For comparable flaw dimensions, the amplitudes and frequencies of the stray fluxes on the pipe outer surface caused by internal flaws are in general markedly lower than those that are produced by flaws on the outer surface of the pipe. Therefore, the sensitivity of the probes to possible internal flaws is used in the known stray flux method to attain a reliable recognition of flaws.
However, this principle fails as soon as a flaw is no longer traversed in perpendicular relation to the test or magnetic field direction so that, as a result of the lower frequency spectrum, this flaw is either no longer detected with certainty because, e.g., the frequency spectrum is only insignificantly lower compared to that of an external flaw, or is classified by mistake as internal flaw and tested in relation to the more sensitive internal flaw threshold.
This has the added disadvantage that external flaws, which may still be tolerated, are detected with too high sensitivity and may then be indicated as no longer tolerable internal flaws, resulting in unnecessary rejection or redundant reworking of the pipes.
JP 62185162A discloses detection of the angular position or shape of a flaw extending from a workpiece surface into the interior by means of stray flux technique. Amplitude signals are here generally detected by two spaced-apart sensors placed perpendicular, evaluated, related to one another, wherein an indication of the shape or angular position of the flaw is derived from the relationship. This document provides no indication as to an evaluation of flaw signals extending at an angle with respect to the test or magnetic field direction and as to how a separation of external and internal flaws can be achieved.